This invention relates generally to mixing and more specifically to methods and devices for mixing dissimilar fluids such as a gas and a liquid or two dissimilar liquids for industrial applications.
The mixing together of dissimilar fluids such as a gas and a liquid or two dissimilar liquids has many industrial applications. For example, the mixing of oxygen and a liquid has applications such as the oxygenation of water for biological purposes and the oxygenation of fuel prior to burning to enhance combustion efficiency. The mixing of air and a liquid may be used in the pulp and paper, textile, and other industries in a process known as dissolved air floatation to separate suspended particulate materials from the liquid. Mixing industrial stack gasses with a liquid such as water is useful for removing environmental contaminants from the stack gasses prior to its release to the atmosphere. The mixing of dissimilar fluids such as, for example, oil and water, has industrial application in the creation of emulsions. Further, an existing emulsion may be separated into its constituent components by mixing it with another gas or fluid such as, for example, methane, which acts as an inhibitor to prevent recombination of the constituent components once they are broken apart.
One particular industrial application of gas/liquid mixing occurs in the pulp and paper industry where black liquor, which is a byproduct of cooking wood chips, often is recycled by being burned as a fuel in boilers. Even though such recycling is economically efficient, an emissions problem arises from the fact that untreated black liquor contains concentrations of Sodium Sulfide (Na2S) that can be as high as 40 grams per liter or more. As a result, when untreated black liquor is burned, the Sodium Sulfide contained therein is converted to Sodium Dioxide (SO2) and Hydrogen Sulfide (H2S), which are known as totally reduced sulfur (TRS) compounds. TRS compounds are extremely harmful to the environment and therefore are highly regulated and may not be released to the atmosphere as a component of boiler stack gasses. Accordingly, black liquor often is treated before being burned in order to reduce or eliminate TRS emissions.
One method of treating black liquor prior to burning has been to mix or agitate it with air in a gas/liquid mixing process. When so mixed, Sodium Sulfide within the black liquor is oxidized in a chemical oxidation/reduction or xe2x80x9creduxxe2x80x9d reaction with oxygen molecules in the air and thereby converted to Sodium Thiosulfate (Na2S2O3). Unlike Sodium Sulfide, Sodium Thiosulfate exists in a stable chemical state and thus does not participate in chemical reactions when the treated black liquor is burned in a boiler. Instead, the Sodium Thiosulfate simply precipitates to the bottom of the boiler, where it is ejected as a smelt.
Prior art industrial methods of mixing gases and liquids in general, and air and black liquor in particular, have involved introducing air in the form of bubbles into black liquor and agitating the mixture to break up the air bubbles and distribute them throughout the liquor. The goal, of course, is that oxygen molecules in the air will react chemically with or xe2x80x9coxidizexe2x80x9d sodium sulfide molecules in the black liquor, rendering them inert during combustion of the black liquor. In one prior art process, such mixing is accomplished with rotating mechanical beaters having blades that impact and cut up the air bubbles while agitating the liquid. The problem with such a system, however, is that there is a natural lower limit to the size of the resulting air bubbles because larger bubbles cannot be cut or chopped to a size smaller than the size of the beater blades. Thus, the total composite surface area of the air bubbles in contact with the black liquor is severely limited. As a result, the probability that an oxygen molecule within an air bubble will come into contact with and oxidize a Sodium Sulfide molecule within the black liquor is reduced.
A further exacerbating problem and limitation of prior art gas/liquid mixing methods in general, and black liquor oxidation processes in particular, arises from the fact that the bubbles that are created by the mechanical beater blades of the mixing apparatus tend not to be distributed evenly throughout the black liquor. Instead, the bubbles, partially because of their relatively large size and partially because of the mechanical nature of the process, tend to agglomerate or concentrate into pockets of bubbles separated by relative voids in the liquor. This further reduces the probability that an oxygen molecule within an air bubble with come into contact with or xe2x80x9cfindxe2x80x9d a Sodium Sulfide molecule and thus reduces the efficiency of the oxidation process. To address this inefficiency, it may be necessary to inject many times the amount of air necessary to oxidize the Sodium Sulfide into the mixer and to increase mixing times substantially to increase the probability of oxidation. However, such a brute force method of increasing oxidation efficiency substantially increases the time, energy, and resources required in the mixing process and thus introduces its own inefficiencies.
A final limitation of prior art gas/liquid mixing methods as applied to the oxidation of black liquor is imposed by the fact that the molecules within the liquor are attracted to each other by weak molecular forces known as van der Waals attraction. This results in the molecules clumping together in mutually attracted groups. In many cases, a Sodium Sulfide molecule that needs to come into contact with an oxygen molecule within a bubble in order to be oxidized may be surrounded within such a group by other molecules within the liquid and thus shielded from contact with a bubble and an oxygen molecule. In these cases, oxidation of the Sodium Sulfide molecule can not occur regardless of the volume of gas introduced or the length of the mixing process. This is due, in part at least, to the fact that the energy imparted to the liquor by mechanical beater blades is far less than that required to break the van der Waals attractions and free trapped molecules. In effect, then, the molecular van der Waals attraction within the liquor imposes a physical limit to the percentage of Sodium Sulfide molecules within black liquor that can be oxidized with traditional gas/liquid mixing techniques.
Thus, a specific need exists for a gas/liquid mixing method and apparatus applicable to the oxidation of black liquor in the pulp and paper industry that overcomes the problems, shortcomings, and limitation of prior art processes. More generally, a need exists for a new and unique method of mixing dissimilar fluids, be they gasses and liquids, dissimilar liquids, or otherwise, that is highly efficient, that results in virtually complete mixing in a short time and with a minimum of required energy and resources, and, in the case of oxidation applications, overcomes the physical limits on oxidation efficiency imposed by molecular van der Waals attraction. It is to the provision of such a method and an apparatus for carrying out the method that the present invention is primarily directed.
For clarity of disclosure and discussion, the present invention will be discussed herein primarily in the context of its application to the oxidation of environmental contaminants such as Sodium Sulfide in black liquor within the pulp and paper industry. Such an application is considered by the inventors to be a best mode of carrying out the invention. It will be understood and appreciated, however, that the method and apparatus of the invention is applicable to virtually any situation where dissimilar fluids are to mixed together for industrial or commercial purposes. For example, the invention is applicable in the pulp and paper industry alone to a variety of processes including micro-mixing prior to gasification, mixing stack gases with black liquor in direct contact evaporators, mixing salt cake and black liquor, pulp drying, sludge dewatering, oxygen de-lignification, pulp bleaching by mixing pulp with ozone or other appropriate gases, and atomization of black liquor prior to its use in a recovery boiler. In the petroleum industry, the invention is applicable among other things to the separation of tight emulsions using micro-mixing and to heavy oil upgrading. Within the food processing industry, the mixing methodology of the invention has application in homogenization, oxygenation, and spice mixing processes. Applications within the environmental industry include oxidation/reduction of liquids or components of liquids, concentration and evaporation, BOD and COD reduction, dissolved air floatation, and fuel aeration. Thus, the discussion of the invention herein within the context of black liquor oxidation should not be interpreted as a limitation of the invention but only as representing a preferred embodiment or application and a best mode of carrying out the invention.
Briefly described, the present invention in a preferred embodiment thereof, comprises a unique and highly efficient method of mixing dissimilar fluids together by mechanically inducing cavitation within the fluids in a controlled manner. The result and goal is to obtain mixing on a microscopic level, uniform distribution of one fluid throughout another, and a breaking of van der Waals attractions between molecules within the fluids. In the preferred embodiment and best mode, the invention comprises a method of oxidizing environmentally hazardous compounds such as Sodium Sulfide within black liquor in the pulp and paper industry by mixing air with black liquor using controlled mechanically induced cavitation. The result is a virtually complete oxidation of the hazardous compounds and thus assurance that environmental toxins are not created when the black liquor is burned.
The methodology of the invention, in the context of oxidizing black liquor, comprises the steps of introducing and entraining air in the form of bubbles into a stream of black liquor to form a mixture of black liquor and air bubbles. The liquor/air bubble mixture is then directed into a hydrosonic mixer, which generally comprises a rapidly spinning rotor disposed within a cylindrical chamber within a housing. The rotor is provided with one or more arrays of relatively shallow holes or bores formed around its periphery. A space, referred to herein as a cavitation zone, is formed between the periphery of the rotor and the cylindrical wall of the housing chamber.
As the mixture of gas bubbles and black liquor passes through the cavitation zone, microscopic cavitation bubbles are continuously generated and collapse within the mixture by the action of the bores on the periphery of the spinning rotor.1 The collapse of these cavitation bubbles creates violent and continuous cavitation within the gas/fluid mixture in the cavitation zone, and the energy of this cavitation acts to break up the air bubbles within the mixture into ever smaller bubbles or units. Since the minimum size of the air bubbles is not limited as in prior art mixers, the air bubbles are reduced by the cavitation into millions of substantially microscopic bubbles. Thus, the total surface area of air bubbles in contact with black liquor is significantly greater than in prior art mixers. The increased surface area increases the probability that an oxygen molecule within an air bubble will come into contact with and oxidize a Sodium Sulfide molecule within the black liquor. Further, because of the relatively violent agitation within the cavitation zone caused by rotor motion and cavitation effects, these microscopic air bubbles are mixed completely and uniformly throughout the black liquor, which further enhances the probability of contact between an oxygen molecule and a Sodium Sulfide molecule. Finally, the energy imparted to the mixture by the cavitation within the cavitation zone is more than sufficient to overcome the van der Waals attraction between molecules within the black liquor. This breaks apart the molecule clumps and frees Sodium Sulfide molecules that may be jacketed or shielded by other molecules within the black liquor. These freed molecules, then, are available to be contacted and oxidized by an oxygen molecule within one of the microscopic air bubbles.
The term xe2x80x9ccavitation zonexe2x80x9d is used herein to refer to the region between the outer periphery of the rotor wherein the bores are formed and the cylindrical wall of the housing chamber. This is where the most intense cavitation activity occurs. It should be understood, however, that cavitation may occur, albeit with less intensity, in regions other than this space such as, for example, in the reservoir or region between the sides or faces of the rotor and the housing. Thus xe2x80x9ccavitation zonexe2x80x9d is used herein to refer to the region of most intense cavitation, but should not be interpreted as an implication that cavitation cannot occur at some level in other regions of the hydrosonic mixer. 
As a result of the creation and uniform distribution of microbubbles and the breaking of the van der Waals attractions, virtually complete oxidation of the Sodium Sulfide component of the black liquor is accomplished within the hydrosonic mixer. (Of course, a small amount of oxidation also may occur outside the mixer such as, for example, in the supply and outlet conduits of the system.) Further, the process can be accurately controlled by selecting rotation rate of the rotor and the amount of air initially introduced into the black liquor such that the complete oxidation is accomplished within a minimum time and with a minimum of energy and required introduction of air. The overall result is a gas/liquid mixing process that is far more efficient, faster, and more effective than is possible with prior art mechanical mixers. Once the Sodium Sulfide in the black liquor has been oxidized, the treated black liquor can be burned in a boiler with minimum of environmental toxins being produced and released to the atmosphere in stack gasses.
Thus, a method of mixing dissimilar fluids such as air and black liquor is now provided that addresses and overcomes the problems and shortcomings of the prior art. The method is highly efficient and effective and results in virtually complete oxidation of target components within black liquor or other liquids. These and other features, objects, and advantages of the methodology of this invention will become more apparent upon review of the detailed description set forth below when taken in conjunction with the accompanying drawing figures, which are briefly described as follows.